The present invention relates to a method and an apparatus for polishing a metal film and, more particularly, to a method and apparatus for polishing a metal film formed on a semiconductor device by a chemical mechanical polishing (CMP) procedure.
It is a common practice with a semiconductor device to form an insulation film (BPSG film) below a wiring on a wafer or similar substrate, form contact holes in the insulation film, and then form a metal film on the insulation film and the walls of the contact holes. Subsequently, the metal film is polished by the CMP procedure. CMP is effected on a polishing pad mounted on a rotatable platen by use of slurry containing Al2O3 or similar abrasive grain and H2O2, KOH, NH1OH or similar acid or base. A specific CMP procedure is taught in, e.g., U.S. Pat. No. 4,992,135.
The metal film has customarily been formed over the entire surface of a wafer in order to increase the effective number of chips. This, however, brings about a problem that the metal film turns round to the rear of the wafer via the circumferential edge of the wafer. The part of the metal film existing on the edge and the peripheral portion of the rear of the wafer cannot be removed by the CMP procedure and causes contamination to occur in the subsequent step. Specifically, when the wafer with such a metal film is positioned with respect to the polishing pad, the circumferential edge of the wafer is enclosed by a ring for preventing the wafer from jumping out. As a result, the metal film on the edge of the wafer cannot be removed even when use is made of a soft abrasive pad. Moreover, although the abrasive turns round to the rear of the wafer via the circumferential edge, the metal film on the front of the wafer is polished at a rate far higher than the etching rate of the metal film on the edge of the wafer. Consequently, the polishing operation ends before the metal film on the edge is fully polished.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 64-55845, 4-167448, 4-233224, 5-275366, 8-124886, and 8-139060.
It is therefore an object of the present invention to provide a method and an apparatus for polishing a metal film formed on a semiconductor device and capable of removing an undesirable part of the metal film present on the circumferential edge and rear of a semiconductor wafer efficiently.
In accordance with the present invention, a method of polishing a metal film formed on a semiconductor wafer has the steps of immersing, before polishing the metal film, the wafer in at least one kind of oxidizing solution for a preselected period of time, and etching an undesirable part of the metal film by use of the oxidizing solution to thereby etch the undesirable part.
Also, in accordance with the present invention, in an apparatus for polishing a metal film formed on a semiconductor wafer, the semiconductor wafer is immersed, before polishing, in an oxidizing solution filling a bath to thereby etch and remove an undesirable part of the metal film.
Further, in accordance with the present invention, an apparatus for polishing a metal film formed on a semiconductor includes a rotating section for sucking, before polishing of the semiconductor wafer, the center of the semiconductor wafer and causing the wafer to spin. A polishing section causes abrasive pads to contact an undesirable part of the metal film deposited on the circumferential edge of the wafer spinning, and polishes and removes the undesirable part with an abrasive being fed to a position where the abrasive pads and the metal film contact each other.